Abstract

We describe the analysis of existing and new maximum-latewood-density (MXD) and
tree-ring width (TRW) data from the Torneträsk region of northern Sweden and the
construction of 1500 year chronologies. Some previous work found that MXD and TRW
chronologies from Torneträsk were inconsistent over the most recent 200 years,
even though they both reflect predominantly summer temperature influences on tree growth.
We show that this was partly a result of systematic bias in MXD data measurements
and partly a result of inhomogeneous sample selection from living trees
(modern sample bias). We use refinements of the simple Regional Curve Standardisation (RCS)
method of chronology construction to identify and mitigate these biases.
The new MXD and TRW chronologies now present a largely consistent picture of long-timescale
changes in past summer temperature in this region over their full length, indicating
similar levels of summer warmth in the medieval period (MWP, c. ad 900-1100) and the
latter half of the 20th century. Future work involving the updating of MXD chronologies
using differently sourced measurements may require similar analysis and appropriate
adjustment to that described here to make the data suitable for the production of
un-biased RCS chronologies. The use of 'growth-rate' based multiple RCS curves is
recommended to identify and mitigate the problem of 'modern sample bias'.

The tree-ring chronologies were constructed using the
CRUST (CRU Standardisation of Tree-ring data) software.
A separate article has been published that describes the processing methods that
CRUST implements, and the full CRUST software (including source code) is now available.

We have created a reduced-functionality version of the CRUST software that is specific to the Tornetrask paper
(i.e. it does the specific processing necessary to construct the tree-ring chronologies, undertake the various
analyses reported in the paper, and to create most of the figures shown in the paper).
This Tornetrask version of CRUST is available in the zip file linked above.
It contains all the raw data, together with executable versions for Linux and Windows.
It also includes the Fortran source code, from which it can be compiled.
We are not able to offer any technical support, but it does include a "makefile" to assist with compiling the software
(though we recommend using the pre-compiled, executable versions if they work on your computer system).